A turbine unit of this type has been disclosed in U.S. Pat. No. 4,179,247. This document emphasizes correctly without doubt, that high precision of guiding and centering can be obtained with a roller bearing, it being understood that the document discloses a ball bearing. Although this document is older than twenty years already, it has had no impact on any practical application. The reason is probably that the turbine housing—in order to house the roller bearing—has such a complicated shape that it could not be reasonably fabricated. In addition the necessary surface treatments on portions which are not easily accessible, would have additionally increased fabrication cost. The measures which are taken in order to allow access to parts which possibly need repair, weaken the housing, and result in an unacceptable lack of operational security. This disclosure—in spite of the likely advantages of a roller bearing—was not sufficiently matured and therefore not suitable for practical application.
It was already known to locate the control elements on the vane shafts at their ends opposite to the nozzle ring, and whereby control levers were used with each extending approximately radially and which comprised one free end. It is additionally known from DE-C-954,551, to provide the control ring with teeth in which a pinion may engage. Other control mechanisms have been proposed which use the effect of interengaging teeth. Also the use of cams in form of slots is known in order to pivot the vanes around their shafts.